Sensors, such as the Nalco 3D fluorometer, are useful instruments for measuring water quality and controlling industrial water treatment systems. Fouling of the sensor due to contaminants in water, however, is a well-known problem. When the fouling potential of the water is great enough, sensors foul so quickly and often that they can become practically useless. An example of a type of water with great fouling potential is wastewater. Depending on the configuration of the sensor, different mechanical approaches have been used to reduce and/or eliminate fouling on critical areas of the sensor.
A variety of sensor designs employing mechanical fouling prevention techniques are known in the art. For example, “probe” style sensors where the measuring system is exposed to the water at one flat end of the probe are often equipped with a rubber wiper designed to wipe away foulants from the face of the probe. Examples of such devices are illustrated in U.S. Pat. Nos. 5,416,581 and 7,341,695. The wiper operates intermittently and must be replaced on occasion. In addition, the motor inside the probe that drives the wiper may fail from time to time, and the seal separating the electronics from the liquid medium can also be a point of failure. Even during normal operation, the presence of a wiper mechanism on an otherwise flat faced probe can provide an attachment point for foulants to begin depositing on the probe.
As illustrated in U.S. Pat. No. 6,678,045, probe-style sensors have also been equipped with ultrasonic transducers designed to vibrate the optical sensor at a certain frequency, or over a range of frequencies. Similar approaches employing ultrasound have been applied to vibrate an instrument with a glass cuvette for optical measurements of a flowing water stream (e.g., U.S. Pat. No. 7,808,642), an optical flow cell (e.g., U.S. Pat. No. 6,452,672), an ultraviolet disinfection system (e.g., U.S. Pat. No. 7,763,177), a steam generator (e.g., U.S. Pat. No. 6,572,709), and fluid filled tubes with closed ends (e.g., U.S. Pat. No. 5,529,635). In these examples, the devices that transmit ultrasound make contact with a solid surface of the sensor and are constantly powered. To prevent breakage of the sensor, these applications employ low power and low intensity ultrasound, which has been found ineffective for preventing or removing fouling of sensors. Further, ultrasound has been applied to clean interior surfaces (see U.S. Pat. Nos. 7,799,146; 5,889,209; 6,977,015).
Other mechanical devices for preventing or removing foulants on sensors exist. For example, pressurized air or water (e.g., U.S. Pat. No. 7,250,302), or pressurized process fluids (e.g., U.S. Pat. Nos. 7,803,323 and 4,385,936) in the form of a jet are intermittently sprayed at the critical area of the sensor surface to remove foulants.
Accordingly, there is a need for a device and/or method for preventing removing fouling of sensors. Desirably, the device and/or method would be effective for use in even the most contaminated fluid. More desirably, the device and/or method would employ high intensity ultrasonic technology without the need for operator intervention.